The present invention relates to flexible twin-lead cables, and in particular to such cables incorporating high-strength textile ribbons. The present invention has application to geophysical explorations.
Geophysical exploration includes the identification of rock strata formations and oil and mineral deposits through the evaluation of seismic reflections. Data from several detectors or geophones are brought together at a central processing station for interpretation.
Typically, large diameter electrical cables with a multitude of pairs of conductor leads have been used to transmit several channels of seismic information independently and simultaneously. The considerable bulk and weight of these cables renders it costly and inconvenient to transport and drag the long lengths required in geophysical applications. Moreover, the manufacturing expense of such cables is a salient factor since miles of cable are often required for each exploratory operation.
More recently, multiplexing, a means of simultaneously sending a number of independent information signals over a common communications medium, has permitted twin-lead cables to replace the more bulky multiple pair cables.
The choice of cable for multiplexed geophysical data transmissions is far from arbitrary. The cable must be able to transmit reliably, often over several miles, informationally dense electrical signals with a wide range of frequency band widths. The 300 ohm twin-lead cable commonly used to connect television antennas has proven suitable in terms of its electrical properties for the geophysical applications. More specifically, Belden Corporation 8285 Permohm cable, as disclosed in U.S. Pat. No. 2,782,251, has proved particularly well-suited for the transmission of multiplexed seismic data.
Unfortunately, even the most sturdy of antenna cables suffer from the physical stresses of being dragged along in long lengths across rough terrain, becoming snagged on rocks and vegetation, and being otherwise roughly handled. Such physical stresses have at times proved sufficient to break the cable. More serious, because less discernible, is the cable elongation resulting from the large longitudinal stresses placed on the cables. The elongation causes the conductors to narrow, altering their capacitance and consequently the impedance of the cable. The resulting anomalies in impedance may render the cable unfit for the reliable transmission of multiplexed geophysical data.
A number of methods are known for improving the resistance of a cable to linear stresses. These include adding high tensile strength textiles as reinforcement for a cable. An example of this textile reinforcement is described in U.S. Pat. No. 4,220,812. Several attempts to adapt prior approaches to the problem at hand have not been entirely satisfactory because the increase in strength has been insufficient, or flexibility has been reduced to an unsatisfactory extent, or the electrical parameters have been significantly altered. The altering of the electrical parameters may be due to the capacitive or dielectric properties of an added strength member, or to the displacement of material currently employed in the twin-lead cable, or both.
It is therefore a primary object of the present invention to provide a cable that is at least twice as resistant to linear stress as currently employed twin-lead cable and which differs negligibly from the latter in flexibility and electrical performance.